The present invention relates to a method and a device which allows an aircraft turbine engine to safely shut down despite the introduction of a large rotor unbalance due to, for example, excessive damage to a fan blade.
An unbalance in the rotor of an aircraft turbine engine during operation creates a rotational load which is transmitted to the turbine engine structure through bearings and bearing supports, causes rotor-to-stator contact, and is then transmitted to the aircraft""s structure. There are two types of unbalance: inherent manufacturing unbalance and accidental unbalance. Inherent manufacturing unbalance are at low levels, although they are not negligible. Accidental unbalance comes mainly from excessive blade damage. This unbalance can be considerable and can result in a rotational load which can be excessive. The engine must be capable of safely shutting down before it damages the aircraft structure. Consequently, a first problem to solve is to maintain the turbine engine in operation, despite the unbalance, at least for a limited time until the engine can be safely shut down without damaging the engine support structure.
Modern turbine engines generally contain a first stage of rotating blades called the fan stage which provides the fundamental propulsion effort, particularly in subsonic turbine engines. These fan blades are very vulnerable to foreign object damage, since they are located at the very front of the turbine engine, because they are thin, of large size and held at one end by the rotor while the other end is free at the rotor""s periphery. Although the damage usually occurs near the free end of the blade, the unbalance it generates can be excessive because of the large size and high rotational speed of the blade. The unbalance in large turbine engines can produce a rotational load on the order of  greater than 200,000 LBS at 6,000 RPM. Therefore, in the presence of such a great unbalance a second problem is to keep the aircraft structure intact.
U.S. Pat. No. 4,289,360 discloses a turbine engine containing a normally rigid bearing support, but which can be released by the breakage of linkage elements under the effect of a strong unbalance. The unbalance can be a result of excessive damage to a rotor blade rotating in a housing with a thick abradable material. The rotor then tends to rotate around its new axis of inertia, which reduces the unbalance and the load that is exerted on the turbine engine support and aircraft structure.
EP 0 814 236 discloses a rigid bearing support system for a rotor of a turbine engine bearing and a frangible connection for reducing load on the engine""s support structure at excessive rotor unbalance. One disadvantage of the system resides in the fact that the frangible connection is subjected to predominately shear load which is undesirable because of the tight dimensional control required between each bolt and bolt hole to insure a repeatable load distribution among the bolts
Accordingly, it is the principle object of the present invention to provide a bearing support system for an aircraft turbine engine, which allows the engine to safely shut down after excessive unbalance is introduced at the fan stage.
It is a further object of the present invention to provide a bearing support system, as set forth above, which includes a frangible link, which fractures in response to excessive unbalance of the rotor.
It is a still further object of the present invention to provide a frangible link, which is subjected to predominantly tensile force, where shear forces are substantially eliminated.
Further objects and advantages of the present invention will appear hereinbelow.
The present invention relates to a method and a device which allows an aircraft turbine engine to safely shut down despite the introduction of rotor unbalance due to, for example, excessive damage to a fan blade of the fan stage of an aircraft turbine engine.
An aircraft turbine engine comprises a rotor having a shaft which rotates about an axis of rotation R during balanced engine operation, a fan stage having at least two fan blades attached to the shaft, a bearing support structure for supporting the shaft for rotation, said bearing support structure comprising a front bearing and a rear bearing, and a first bearing support and a second bearing support for securely attaching the front bearing and the rear bearing to the aircraft turbine engine""s support structure, respectively. In accordance with the present invention, the first bearing support includes a joint located at an axial distance xe2x80x9caxe2x80x9d from the front bearing. The joint includes a frangible linkage which is designed to substantially eliminate shear forces on the frangible linkage so that the frangible linkage is subjected to predominantly tensile force.
The present invention further relates to a method for sensing predetermined excessive operating unbalance of the rotor and thereafter decreasing load transfer to the aircraft turbine engine""s support structure, which includes substantially eliminating the transfer of shear forces to the frangible linkage and breaking the frangible linkage at a tensile force corresponding to the predetermined excessive operating unbalance of the rotor such that the support of the rotor by the front bearing is lost and the shaft rotational action is changed to decrease load transfer to the engine""s support structure.
The present invention provides an improved bearing support system which allows a turbine engine to shut down in a safe manner after experiencing an unacceptable operating unbalance of the rotor.